1. Technical Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to a light printer which is capable of forming images in a composite mode wherein images are formed by overlaying images on the same surface of a sheet and in a double-sided mode wherein images are formed on both sides of the same sheet.
2. Description of Related Art
In an image forming apparatus such as a printer, copying machine, and the like, electrophotographic image forming apparatus has heretofore been well known for forming images wherein an electrostatic latent image is first formed by exposing a photoconductor with an image and then the electrostatic latent image is developed by toner to transfer the image onto a copy sheet. As an exposure medium, it is well known that a laser and LED elements can be controlled for an exposure process based on the image data produced from an original by reading the original image or with the use of a personal computer, word processor or the like, besides an optical system exposure assembly in which the reflection of an original image is irradiated by using the optical system comprised of lenses and reflectors. In a laser exposure process, a rotary scanning system has been put into practical use as an exposure system in which a modulated laser beam is deflected by a rotating polygon mirror for a scanning operation, and in an LED exposure process, a fixed scanning system has been used as an exposure system wherein an LED array is provided with a multiplicity of LED elements closely arranged with a selective turning on of each LED.
On the other hand, as image forming modes, there are provided a composite mode for forming images on one side of a sheet overlaying images and a double-sided mode for forming images on both sides of a sheet, in addition to ordinary mode in which a single image is formed on one side of a sheet. For such composite and double-sided modes, an apparatus is well known which is arranged for repeatedly carrying out an image transfer and fixing process more than twice on the same sheet. However, in such composite and double-sided modes wherein a transfer and fixing process are repeatedly performed a plurality of times on the same sheet as described above, the sheet is liable to expand or contract since the sheet is heated or pressed by a first round of a fixing process, and when an image is transferred and fixed with the same magnification at a second round of a fixing process, there arises slippage between images especially in a composite image forming process, and a proper image cannot be obtained.
For instance, Published Unexamined Japanese Patent Application No. 177567/1987 discloses an image forming apparatus which is capable of solving the above problem, wherein the contraction of a sheet which arises under the composite and double-sided modes is rectified by delaying the sheet transport speed at the time when a second image is formed.
In U.S. Pat. No. 4,789,879, there is disclosed an alternative solution to the problem. In a copying machine adapted to project and expose an original image onto a photoconductor, it is arranged to correspond with the expansion or contraction of the sheet by scanning an original image with scanning means which is comprised of lenses and mirrors when a copying operation is carried out under a composite or double-sided mode wherein the copying magnification is varied according to the change is speed of scanning and the movement of the lenses. These methods may be applied only to a copying machine which is adapted to scan an original image with scanning means as described above, however, they cannot be applied to an ordinary light printer.
In each of the prior arts described, the slippage of composite images is rectified by predetermining the amount of contraction of the sheet when the images are formed thereon. However, the rate of expansion and contraction of a copy sheet caused by the fixing process is varied according to the fixing condition of the image forming apparatus, the kind and area of the sheet, the textured direction of fabric, and the percentage of water content of the sheet, ambient temperature, humidity and the like. Especially, the percentage of water content, ambient temperature and humidity which largely vary by themselves are factors that will largely vary expansion and contraction of the sheet, and it is very difficult to settle these conditions in view of the maintenance and daily handling of the sheet. Accordingly, when the expansion and contraction of an image is controlled to a high accuracy corresponding to the expansion and contraction of the copy sheet, it will become more difficult to correspond with the actual rate of expansion and contraction of the sheet.